Fix broken tag autocomplete on multiple pages.

Mark all tag <input>s with a `data-autocomplete` attribute, instead of
hardcoding a list of html IDs to autocomplete in javascript.

This way should be less error prone. It fixes autocomplete in several places:

* Autocomplete for the search box on /posts didn't work in the
  responsive layout. This was because /posts has two search boxes that
  both have the id `tags`: one in the normal sidebar, and one in the
  responsive tag list. $("#tags") only initialized autocomplete on the
  first one.

* Autocomplete didn't work on the aliases or implications pages. This
  was due to selecting the wrong html ids.

app/views/posts/partials/common/_search.html.erb

@@ -6,7 +6,7 @@
 <section id="search-box">
   <h1>Search</h1>
   <%= form_tag(path, :method => "get") do %>
-    <%= text_field_tag("tags", tags, :size => 20, :id => tags_dom_id) %>
+    <%= text_field_tag("tags", tags, :size => 20, :id => tags_dom_id, :data => { :autocomplete => "tag-query" }) %>
     <% if params[:raw] %>
       <%= hidden_field_tag :raw, params[:raw] %>
     <% end %>